1. Field of the Invention
One aspect of the present invention relates to at least one of an image forming apparatus and an image forming method using the apparatus.
2. Description of the Related Art
An image forming apparatus is known which is represented by a tandem-type laser beam printer. In such an image forming apparatus, image forming parts that are all different from one another with respect to four colors (black, magenta, yellow, and cyan) and a light exposure device for emitting an exposure light beam for the color of an image formed by each image forming part are used to overlay toner images on a paper sheet directly or on an intermediate transfer belt, thereby forming a color image. Herein, a light exposure device of an image forming apparatus according to a related art will be illustrated.
FIG. 1A is a plan view illustrating a principal configuration inside a light exposure device of an image forming apparatus according to a related art. FIG. 1B is a side view illustrating a principal configuration inside a light exposure device of an image forming apparatus according to a related art. As referring to FIG. 1A and FIG. 1B, laser light beams 140BK, 140M, 140C, and 140Y as exposure light beams of respective colors are emitted from laser diodes 240BK, 240M, 240C, and 240Y as light sources, respectively, in a light exposure device 110. The emitted laser light beams 1408K, 140M, 1400, and 140Y are reflected from a reflecting mirror 230, pass through f-θ lenses 250R for adjusting a main scanning magnification and folding mirrors 250M provided for the respective colors to adjust the light paths thereof, and subsequently scan faces to be scanned which are surfaces of photoconductor drums (not illustrated). Herein, BK, M, Y, and C behind hyphens provided for 250R and 250M indicate optical systems for black, magenta, yellow, and cyan colors, respectively.
The reflecting mirror 230 is a hexahedral polygon mirror and is capable of rotating so as to conduct a scanning operation of one line with an exposure light beam in a main scanning direction per one face of the polygon mirror. Scanning is conducted by one polygon mirror for four laser diodes as light sources. The exposure light beams are divided into two set of colors, laser light beams 1408K and 140M and laser light beams 140C and 140Y, and opposing reflecting faces of the reflecting mirror (polygon mirror) 230 are used to conduct scanning, whereby it is possible to conduct simultaneous light exposure for four different photoconductor drums (not illustrated).
A synchronization detection sensor 260 is arranged outside an imaging area in a main scanning direction, detects the laser light beams 140BK and 140Y for each scanning operation of one line, and adjusts the timing of start of light exposure for image formation. The synchronization detection sensor 260 is arranged at the side of the f-θ lens 250R-BK, and hence, the laser light beam 140Y is incident on the synchronization detection sensor 260 via the folding mirrors for synchronization detection 250M-Y1 and 250M-Y2. The laser light beams 140M and 140C are not capable of adjusting the timing of writing by a synchronization detection sensor, and hence, the timing of start of light exposure for magenta and the timing of the start of light exposure for cyan coincide with the timing of start of light exposure for black and the timing of start of light exposure for yellow, respectively, thereby adjusting the positions of images of the respective colors.
Meanwhile, if the positions of overlaid toner images of the respective colors are displaced subtly, it may be impossible to obtain a color image stably. Hence, a pattern for positional displacement correction is formed for each color in an image forming apparatus according to a related art and the positions of toner images of the respective colors are detected by a detection device such as a TM sensor (toner marking sensor), thereby conducting positional displacement correction to overlay all the four colors at an identical position.
When positional displacement correction is conducted, four color images may generally overlay at an identical position and the amount of color displacement may approach zero. However, as time has passed after conducting positional displacement correction, the amount of color displacement may increase due to various factors. In particular, displacement of the position of a reflecting mirror due to a temperature rise inside a light exposure device may often be a main factor of an increase in the amount of color displacement. Although a reflecting mirror is fixed by using a screw or an adhesive material in a light exposure device, the shape thereof may be changed or the shape of a supporting member may be changed with a temperature rise, and hence, the inclination thereof with respect to the light path of an emitted light beam may be changed readily. Due to such a change in the inclination, the amount of color displacement may increase.
In order to correct an increased amount of color displacement, it may be necessary to conduct positional displacement correction in which a pattern for positional displacement correction is image-made and detected. However, a time period of 10-20 seconds may usually be required to conduct positional displacement correction and such a time period may be down time for a user.
For a method for reducing such a time period to conduct positional displacement correction, a PD with a non-parallel shape may be used as a sensor (synchronization detection sensor) for adjusting the timing of writing in a light exposure device. A PD with a non-parallel shape has a shape that is perpendicular at one end portion and has obliqueness at the other. As a laser light beam passes through a PD with a non-parallel shape, the timing of passage through a perpendicular end portion is constant independently of the inclination of a reflecting mirror but the timing of passage through an end portion having obliqueness is changed depending on the position of an exposure light beam in a sub-scanning direction. The difference between the results of detection at such a perpendicular end portion and an end portion having obliqueness is used to calculate an amount of positional displacement of light exposure in a sub-scanning direction and an amount of sub-scanning color displacement on an image is calculated for correction from such an amount of positional displacement of light exposure (for example, see Japanese Patent No. 2858735, Japanese Patent No. 2642351, and Japanese Patent Application Publication No. 2005-221824).
However, a problem may be that a PD with a non-parallel shape as used in a related art may be expensive, and accordingly, the cost of manufacturing an image forming apparatus may be increased.
Also, it may be necessary to arrange PDs with a non-parallel shape in the light paths along which all the laser light beams of four colors are reflected from folding mirrors and reach photoconductor drums, and accordingly, a number of mirrors may be required to establish light paths of laser light beams. A problem may be that the cost of manufacturing an image forming apparatus may also be increased due to such a matter.
Also, if deformation of a light exposure device is caused due to a temperature rise inside the light exposure device or the like, a problem may be that the distances among respective PDs with a non-parallel shape may vary, whereby an amount of positional displacement of light exposure in a sub-scanning direction may not be correctly calculated and an error may be caused in a correction value for the amount of positional displacement of light exposure. In such a case, it may be impossible for a PD with a non-parallel shape to detect a deformation of a light exposure device, and hence, it may be impossible to correct an error in a correction value.